drm: create mode_config idr lock

Create a separate mode_config IDR lock for simplicity.  The core DRM
config structures (connector, mode, etc. lists) are still protected by
the mode_config mutex, but the CRTC IDR (used for the various identifier
IDs) is now protected by the mode_config idr_mutex.  Simplifies the
locking a bit and removes a warning.

All objects are protected by the config mutex, we may in the future,
split the object further to have reference counts.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>